Antenna system for short waves



2-SheesSheet 1 INVENTOR Nov. l, 1949. L. c. VAN ATTA.

ANTENNA SYSTEM FOR SHORT WAVES Filed Ooi. 25, 1943 L. c. VAN ATTA y2,486,620

ANTENNA SYSTEM FOR SHORT WAVES N ov. l, 1949.7

Filed Oct. 25, 1 945 2 Sheets-Sheet 2 INVENTOR LESTER C. VAN

ATTA

Patented Nov. 1.949g

Lester C. Van. Attaj WinchesterMass.- ,assigner, by mesne assignments,to the United States'of` America as represented by. the Secretary of theNavy Application Octoberf 25, 1943-, SerialNoz 507,585

15 claims. l

This invention relates to radio antenna systems, andK particularlyantenna systems for operation on micro wave lengths, including anantenna, a smalslfreecting element located. near and forwardly of saidantenna and a large reflector at a greater distance from said antennaand locatedl rearwardly of it.

Directive antenna systems for micro wave lengths, which is to say forwave lengths less than about 50 centimeters, have in the past4 madegreatv use of parabolic reflectors` which are able to give a high degreeof directivity. Parabolic reflectors are also fairly readily adapted foraiming` in different directions and may when suitably mounted be movedabout rapidly and convenientlyI without appreciable distortion of thedirectivity of the beam. produced. In order to realize the fulladvantage of a. parabolic reiiector it is important that the paraboloidbe properly illuminated at all times,- which isto say that the antennasystem associated with the parabolic reflector should radiate as much aspossible in the direction of the paraboloid and as little as possible inother directions, especially directions at wide angles to the axis ofthe parabolic reflector.

A- metallic sheet reflector in. front of the antenna is sometimes usedfor the purpose of cutting off all direct forward radiation of theantenna and reflecting such radiations back into the parabolicreflector. Various shapes of sheet metal reflectors have been used. Acircular disk somewhat less than one wave length in diameter has beenfound to be the most effective and suitable of such arrangements. beplaced approximately a quarterewave length in front of the antenna, andpreferably slightly more than a quarter-wave length, for instance about0.27 wave length. In such an arrangement the effective center ofradiation of a combination of a dipole antenna and a reflecting diskusually lies midway between the dipole antenna and the reflecting disk.In some cases ity may lie closer to the reflecting disk than such'inidpoint. The location of the effective center of radiation can bedetermined experimentally with sufficient accuracy for antenna designinformation. Such determination may make use of measurements and plotsof phase patterns in the neighborhood of the system, from which thelocation of the effective center can be deduced.

An ultimate object of this invention is torv provide a steerabledirective antenna system for microwave lengths providing a moreconcentrated and powerful beam than has heretofore Such a disk should f2.. been possibile for` a given transmitter power andparaboloidreflector. dia-meten Particular gobe'cts of thisinventionvincludef. production of an antenna` with an: improved radiation pattern(with regard tof side. lobes, beam. width, etc.), the provision;y of an4antenna which is` properly matched toA the. feed: line associatedtherewith sothattherefwill be maximum transfer of energy to: thetransmitted.` beam, and the provisionpf means for. illuminating aparabolic reectoiwith microwavefradiatiommore effectively than in theprevious. practica.

Thev invention is illustrated. in; the accompanying4v drawings in:which:

Eig.;` 1 is a side; View;` partly in section,l showing'` thepreferrediformf of antenna systems in` accordance with= this:invention;.v

Fig. 2.isI a sectionalview showing in detail the construct-ion of theforward part ofthe antenna system of Fig..y 1;.

Fig.. 3. a. perspective view of another form ofi antenna'. system.according to this invention, the parabolic; reflector. being omitted forcon,- venience, and

Figs; 4,. 5,1. and' 6k are diagrams ofV radiation patternsfthatzarebelieved to correspondv to certain: antenna. systemsherein describedandi which are providedifor elucidation' of the principlesberlievectl tocontrol; the operationsv of antenna sys'- tems. according. to.y thisinvention.

I have' found. that highlyy satisfactory illumination. of. parabolicrelectorsffor the. purpose of forming a concentrated bearrr of radiationcan be obtained witlr ai parasitic dipoler reflector, sometimes knownas` a: dummy.,-. infront of' the an:- tenna properinstead off a s'heetor disk reflector provided the' dummy is:4 properly spaced A though fairconcentration ofl the: radiated beam can be achieved with such; a dipolelocated one quarter-wave: length ini frontl of the antenna dipole; thedirectionalproperties-of the beam are greatly improved if the parasiticreflector dipole is spaced' considerably closery to the antenna proper:than; one quarter-wave length; and.l preferably oneeig'htliewave length.This is. especia-ily true if: at the samej time the distancek betweenth'e midpoint of the antenna-parasitic'- 3 in the drawing the parasiticreflector 3 is supported on a projection 4 inside a thin bulb 4a of highquality insulating material, such as polystyrene, which serves toexclude moisture and dust and may also serve to maintain atmospheric orhigher pressure to prevent the occurrence of corona discharges when theapparatus is used at high altitudes. The antenna dipole I, 2 is fed by acoaxial feed line 5 and is provided with rounded arms I and 2, thesearms being rounded primarily to reduce corona. It will be noted that thedipole I, 2 is at right angles to the axis of the feed line 5. Thisconfiguration is preferred because it permits introducing the feed linethrough the vertex of the parabolic reflector and supporting the feedline and dipoles at the said vertex, thus keeping the feed linesymmetrical with respect to the parabolic reflector, which is a greatadvantage in obtaining a concentrated beam of radiation and a gooddirectional radiation pattern. If for some reason the advantagesresulting from the symmetrical location of the feed line with respect tothe parabolic reflector can be dispensed with, other advantages of thisinvention can of course still be realized with other well-knownarrangements of the feed line with respect to the dipoles and theparabolic reflector.

In order to provide better support of the feed line and antenna systemfrom the vertex of the parabolic reflector the outer conductor of thefeed line 5, as shown on Fig. 1, is progressively thickened as itapproaches the parabolic reflector, which is shown at 6, so that theouter surface of the feed line 5 has a flaring shape. The feed line andits associated antenna system is fastened to the parabolic reflector 6by a suitable clamping or bclting device, a simple form of which isshown on Fig. l in the form of a shaped washer 8 and a nut 9 cooperatingwith a shoulder Ill forming part of the feed line body; or the feed linemay enter through a slot or hole in the reflector when the latter is tobe moved with relation to the antenna proper. The end of the feed lineon the outside of the parabolic reflector 6 is shown threaded at I I forreceiving a coupling for the purpose of connecting it to a furthersection of line or a rotating joint forming part of the transmission andreception system with which the antenna system is designed to beemployed.

The effective center of radiation of the system comprising the dipole I,2 together with its parasitic reflector dipole 3 is the midpoint betweenthe centers of the respective dipoles. Heretofore it has been customaryto locate the effective center of the illuminating source at the focusof the parabolic reflector for the formation of a directive beam ofradiation, irrespective of the relation between the focal length and thewave length in question. I have found that it is more important tolocate the effective center of radiation at a point separated from thevertex of the parabolic reflector by an integral number of half-wavelengths than to take care to locate the effective center of radiationexactly at the focus. I therefore preferably locate the effective centerof radiation at that number of half-wave lengths from the vertex of theparabolic reflector which brings the effective center of radiationnearest to the focus, which is to say that in the apparatus of Fig. 1the dimension a should be equal to where i is the Wave length and' 11.is the whole number which brings the expression nearest to the focallength of the parabolic reflector.

In cases in which the focus falls approximately half-way between twohalf-wave points, the effective center of feed should be located at thathalfwave point which more nearly corresponds to the subtending of theoptimum solid angle at the effective center by the aperture of theparabolic reflector. This will depend on the directivity of the feed(the feed being the antenna dipole and auxiliary reflector combination)and on the shape (i. e. focal-length-diameter ratio) of the parabolicreflector. For example, if the parabolic reflector is initially somewhattoo deep, the first half-wave point beyond the focus should be chosen.The matter of the most desirable paraboloid shape for antennas of thisinvention is discussed further below. In any event, of the points atintegral numbers of half-wave lengths from the vertex of the parabolicreflectors, one of the two falling closest to the focus is generally tobe chosen for the effective center of radiation. If this is done, sucheffective center will in every case be not more than a half-wave lengthfrom the focus. If the nearest point is chosen, the effective centerwill be at most a quarter wave length from the focus.

Where it is possible to do so. I provide the antenna system with aparabolic reflector the focal length of which is equal to an integralnumber of half-wave lengths of the radiation in question.

,It is not always convenient to do so, however,

because templates for the manufacture of parabolic reflectors areexpensive and it is consequently not economical to manufacture suchreflectors in many different focal lengths. The adjustment justdescribed of the dimension a, referring to Fig. 1, is especiallyimportant where close spacing such as spacing of the order of one eighthwave length, is used between the antenna dipole I, 2 and the reflectordipole 3, for reasons more fully explained below.

As is more fully explained below in connection with Figs. 4, 5 and 6,the importance of the above considerations in locating the antenna andits associated auxiliary reflector with respect to the paraboloid vertexdepends on two factors: (a) the proper phasing of direct forwardradiation with radiation reflected from the parabolic reflector and (b)the obtaining of a minimum antenna input impedance in order to match thefeed line as well as possible to the antenna. It is important that theinput impedance of the antenna system be kept low in order that thisimpedance may be capable of being matched to the feed line by means of asleeve type transfoarmer of practicable dimensions. If the transformeris required to provide a large transformation ratio, excessivefrequency-sensitivity may result. When a given system, consisting of afeed line, an antenna and an auxiliary reflector, is once adjusted forimpedance match at a point near the focus of a parabolic reflectorlocated on the axis of the latter and an integral number of half-wavelengths from the vertex, it is substantially matched for other suchhalf-wave point locations of the same parabolic reflector and alsosubstantially matched, assuming a standing wave power ratio of 1.5 canbe tolerated, for such half-wave point locations in other parabolicreflectors, irrespective of the focal length of the latter, providedthat the original match was obtained in a reflector having a focallength of 1.5 wave lengths or more and that the other reflectors inquestion also have focal lengths of at least 1.5 wave lengths. Forsmaller reflectors, the match depends on the focal length. For the largereflectors, the possibility of`4 change of' reflectors withoutrematching holds also for an-v tenna feeds matched fory locations otherthan the half-wavepoints, provided the location relative to thehalf-wave point is maintained when the reflector is changed.

llhus by the use of the relations here disclosed an antenna feed may bedesigned for effectively illuminating any of a Wide variety ofbeam-forming parabolic reflectors.

Further features of the preferred form of antenna are shown in detail onFig, 2. As illustrated in that ligure a detachable coupling is providedin the feed line 5. at some distance in back of the antenna in orderthat the antenna may be replaced or removed for adjustmentwithoutrunfastening the feed line 5 from its support at the vertex ofthe parabolic reector (not shown on Fig. 2). The coupling includes athreaded tightening sleeve l5, a threaded bushing llmounted on the outerconductor il of the eed line 5, a gasket i8 made of a compressibleelastic material such as neoprene, a split steel friction ring i9 and aconnector plug 2f! fitting intoA recesses 2l and 22.` in the respectiveinner conductors 23 and 24 of the two portions of the feed line.Insulating beads or spacers 25 are shown in the feed line 5 formaintaining the relative positions of the inner conductor 23 and theouter conductor l1. 2G forms the outer conductor of the section ofcoaxial line between the coupling just described and the antenna. It maybe conveniently made of machined brass. It is provided with a shoulder2l and flange 28 which serve respec- 1" tively for mounting, a chokesleeve 29 and for holding in place the inner member or cup 33 of thepolystyrene protective bulb about the antenna system. A gasket 3|v ofcompressible material is provided between the flange 28 and the l' cup3l) for the formation of an airtight seal between the polystyrene bulband the tubular conductor 26. A metallic sleeve or cup 32 is threaded onthe outside of the tubular conductor 26 so that its mouth ts over theoutside of the neck f of the cup 3G, thus holding the cup 30 firmly inplacel and at the same time providing a radio frequency choke adapted toinhibit the formation of standing waves on the feed line 5. A

lock nut 33 is provided for holding the cup 32 dimension b on Fig. 2)must be carefully adjusted. This length should be such that aquarter-wave length oscillation can be maintained in the cavity formedby the conducting parts, which is to say that the cavity acts as aclosed pipe type of resonator. the polystyrene insulation the length ofthe dimension b is considerably less than the quarterwave length ofoscillations of the frequency in question radiating in the open air. Itcan however be calculated by known means from the dimensions of thestructure and the characteristics of the dielectric, or it may bedetermined experimentally. A more specific value of this dimension isgiven below together with values of certain other dimensions which havebeen found suitable for a particular range of wave lengths in anapparatus of the particular configuration of Fig. 2. If the apparatus isto be used at high altitude where the air surrounding the antenna systemon the outside of the cup 30 is at a pres- The tubular element e Becauseof the presence of i:

sure conslderably.belowy normal atmosphericrpresf-f sure, itmaybeadvisabl'elto. cement the` cup130 to the structure 32 wheny theapparatus is as! sembledf to: prevent. the occurrence' of corona. intheresonator formed: by thestructure 32 onaccount of theioccurrenceof"narrow air spaces ins side the structure 32.

The'sleeve- 29,1, like the structure 32, cooperates with the tubularconductor 26 to` provide ares.- onatoronI the outside of the conductor26' which acts to prevent the occurrence of standing: Waves on theoutside ofthe conductor 26 and generallyr on` the feed line 5'. Thisresonator isagain.` a quarter-wave resonatorwith its open. end directedtowards the.A antenna. The depth of. the. cavity should again.correspond to an. electrical quarter. wave length, which in. practice isslightly less than a quarter of the- Wave length in open4 air ofradiation of corresponding frequency, the variation from the` exactquarter-wave length. being probably due toend effects in. thecavity;

The resonator formed: bythe sleeve 29 maybe regarded as a choke tosuppressstanding waves on the outside ofy the feedl line or itmaybeconsidered as a balancing resonator providing a transitionv (i. e.acting as av transf-ormer, in a sense)` between the unbalancedy rearportion feed line (one conductor being at R1 F; ground po.- tential) andthe balanced portion feed` linel near the antenna where both conductorshaver R. F. voltages. withy respect to. ground'. In` this sense theresonator permits the forwardy end portion of the line 5i tobehave as abalanced line While theA rear portion remains unbalanced as required fornormal operation. The distancebetween the mouth of the choke sleeve 2aand the antenna l, 2 is quite important since it affects the impedanceoffered by the antenna system to the feed line. In general the chokeshould be close to the antenna, preferably considerablyl less than aquarter-wave length, and as close as the danger of corona dischargeswill permit under the desired conditions of operation. The exact spac'ing is not quiteA so critical for small differences in wave length assome of the other distances here mentioned and the effect of thisspacing upon the antenna impedance may be compensated for or taken intoaccount in selecting the proportions for the quarter-wave matchingtransformer provided as hereinafter described on the inner conductor 24.

The dipole arm- I, formed in a rather bulbous shape for the mitigationof corona and for the reductionof frequency sensitivity, is mounteddirectlyy on the end of thel tubular conductor 26 as shown, the tubularconductor 2E being cut away on the opposite side at an angle of about 35(see also` Fig. 1) in order to allow ample clearance for the dipole arm2, which is fastened upon a rounded end piece 35 threaded on the endofthe inner conductor 24 of the feed line in such a manner as to embracethe conical insulating bead 36. The dipole arm 2 is preferably of thesame shape and size as the dipole arm l, in order to provide as even aconfiguration of the potential gradient as possible and thereby inhibitcorona discharges. The electrical center of the dipole antenna I, 2 isnot exactly on the axis of the feed linel 5, so that it is advisable toalign the reflector dipole 3 slightly olf-center with respect to theaxis of the feed line 5 so that it may be symmetrical with respect tothe midpoint of the dipole I, 2 as shown on Fig. 2. It is not certainthat this slight asymmetry results in a similar asymmetry of theeffective center of radiation,

because the latter is also affected slightly by the` presence ofstanding waves between the mouth of the choke sleeve 29 and the antenna,which standing waves will-in general not be symmetrical 'with respect tothe axis of the feed line, producing an effect which may well outweighas well as counteract that produced by the mechanical displacement ofthe dipole with respect to the axis of the feed line. Thus for allpractical purposes the antenna system shown in Fig. 2 acts as if its feffective center of radiation were on the axis of the feed line andhalfway between the axes of the dipole I, 2 and the dipole 3. The lengthof the dipole 3 should approximate an electrical half-wave length and ispreferably somewhat longer than the dipole I, 2. The preferred physicaldimensions, in terms of the free space wave length, are given in thetable below.

Keying arrangements 3i' are provided for maintaining the insulating bead36, which is preferably made of high quality polystyrene, in fixedrelation to the inner and outer conductors of the feed line. The bead 36is also preferably provided with a longitudinal groove (not shown) atsome point on its outer periphery for the purpose of allowing.communication between the atmosphere in the feed line and the atmospherein the bulb or pressure vessel surrounding the dipoles. Thus at the sametime the antenna system and the feed line may be maintained atatmospheric or superatmospheric pressure for the prevention of sparkingand corona. It is to be noted that the end piece 35 is provided with arounded end surface the shape of which contributes to the prevention ofcorona discharges.

The inner conductor 24 which is connected to the dipole arm 2 throughthe conducting end pie-ce 35 is provided with a thickened portion 38,the length of which is shown on Fig. 2 by the dimension d. This lengthis usually approximately a quarter-Wave length of the oscillations inquestion, although other lengths may be used provided a suitablecoordination of the Spacing between the thickened portion and theantenna is made in accordance with principles well known in the art. Theamount by which the diameter of the inner conductor 24 is increased forthe length of the thickened section 38 depends not only upon thedimensions of the antenna I, 2 and the position of the choke sleeve 2Qbut also rather considerably` upon the distance between the thickenedportion 38 and the antenna. The desirable distance between the thickenedportion 38 and the antenna will be affected somewhat by the nature andconfiguration of the insulating bead 3S. With proper adjustment of thislength (the dimension e on Fig. 2) the necessary thickening of thequarter-wave matching transformer 38 may be very small, such as onehundredth of an inch additional radius. In general it sho-uld .be notedthat the antenna impedance is affected by variations in spacing betweenthe antenna dipole and the reflector dipole and also by nearbyreflecting objects in the path of the beam cast by the parabolicreflector, so that the matching transformer 38 should be designed withregard to the antenna impedance as it .occurs in the service position ofthe antenna system. The antenna system of Figs. l and 2 constructed asherein described and mounted without obstructions in the path of thebeam cast by the parabolic reflector has an input impedance of not verymuch greater than the characteristic im.. pedance of the feed line 5which may be about ,50 ohms, so that only a small thickening of theinner conductor 24 will suice to provide a suitable impedance match.

In an apparatus of the particular type shown in which apparatus theinner diameter of the outer feed line conductor was between 1/ and wavelength, good results have been obtained with the distance e almostexactly equal to a quarter of the free-space wave length. The electricalwave length in question is greater and may be nearer a half than aquarter of the wave length on account of the insulating bead 36. Theantenna input impedance at the point of feed is slightly inductive inthe particular apparatus shown in Figs. 1 and 2 when constructed asherein described for wave lengths between 9 and 10 centimeters, Ibutthis effect is to some extent compensated by the capacitive loadingresulting from the location of the insulating lbead 36 between the innerand outer conductor.

The design of matching transformers consisting of a quarter-Wave sectionof transmission line of a characteristic impedance different from thatof the line to which it is desired to match a particular load impedanceis well understood. The determination of the impedance of the load isnot always so simple, particularly at microwave frequencies. Techniqueshave recently been evolved however for making this determination, andone of the most practical of these makes use of the measurement of thestanding wave ratio resulting from the connecting lof the loadimpedances in question to a line of known characteristic impedance. Sucha determination will provide a basis for calculating the desireddimensions of matching transformer and its location in the line. Theaccuracy of the calculation can then -be checked by another measurementof standing waves with the transformer in the line. The nature of theerror will indicate the proper direction for correction of thedimensions of the transformer.

In general, the characteristic impedance of the quarter-wave matchingsection of line should be equal to the geometric mean of theIcharacteristic impedances of the line and of the load, as is well knownin the art. The desired characteristic impedance may be obtained in thequarter-wave section Iby varying the dimensions of the conductors or thedielectric constant of the medium (i. e., -by varying the medium)between them, or both. The characteristic impedance' Zo of a coaxialline depends on the' crosssectional dimensions of the line as follows:

where b is the inner diameter of the outer conductor and a is the outerdiameter of the inner conductor.

The polystyrene cup 3D is closed by another cup-like mem-ber 40preferably made lof the same material to form an airtight vessel or bulbabout the dipoles. The cups 30 and 40 are put together by aclose-fitting joint I where they are preferably cemented together -bypolystyrene cement. Such a cemented joint will hold at all reasonablepressure differentials between the inside and outside of the bul-b. Noobservable effect on the radiation pattern of the antenna system has yetbeen traced to the presence of the polystyrene enclosure on antennasystems constructed in accordance with Figs. l and 2 with reasonablythin bulbs. The cup 40 is provided with a central protuberance 42 uponwhich is mounted a meri ber 43made of similar insulating material whichis adapted to hold in place the reflector dipole 3 in cooperation with akingpin 44. The member 43 is adjusted and dimensioned suitably forholding the reflector dipole 3 at the proper spacing from the antennadipole I, 2 for the wave length in question.

In order to give complete details concerning the preferred form ofantenna for the purpose of illustrating more fully the way in which theinvention is best carried out there are tabulated below certaindimensions relating to the apparatus of Fig. 2 which have been foundsuitable for operation at wave lengths in the microwave range. Whenvariation of one or more of these dimensions is contemplated it shouldbe remembered that these dimensions are in general interdependent andthe performance with respect to the other dimensions should be checkedif one of these dimensions is varied substantially. The dimensions givenbelow relating to the matching transformer 38 have been experimentallychecked only for la narrow range `of shorter wave lengths, but valuesfor other wave lengths in the microwave range are not likely to be muchdifferent, in terms of the wave length, for a similar antenna structure.Since it is convenient within the aforementioned range of wave lengthsto keep constant the position of the mouth of the choke sleeve 29,changes in wave length of operation and corresponding changes of thedimensions given below in accordance with wave lengths may require afurther adjustment of the matching transformer diameter and locationbecause ofv a change of antenna impedance. The dimensions given beloware expressed in terms o-f the free space wave lengths :and of courseapply only if substantial changes in the configuration of spacinginsulators such as the bead 36 and the like are avoided. Where thedimension given is a range of values, the first value given represents avalue which has been found successful at shorter wave lengths, and; thedimension last given represents a value that has been found successfulat longer wave lengths. The values relating to the matching transformerwere obtained with the former values of the other dimensions. Thevariations are at least partly explained in that the feed line diametersand the choke spacing were held constant while the other dimensions andthe frequency were varied.

Table lndlca- M agnitude' in Dimension tion on Free Space Fig. 2 WaveLengths Depth of polystyrene-filled choke b 0.128 Depth of inner choke c0. 23 Length of'matching transformer d 0.248 Position of matchingtransforme e 0. 249 Length of parasitic dipole f 0. 45 to 0. 44 Lengthof antenna dipole 0.408 Dipole arms h 0.161 to 0.167 Spacing betweendipoles 1 0.122 to 0.127 I. d. of outer conductor of Ieedline 0.157 to0.134 O; d. of inner conductor of feed line-. 0:070 to 0. 059 Diameterof matchingtransformer 0. 084

Fig. 3 shows another form of antenna according to this invention whichis suitable for use under conditions in which the protective polystyrenebulb shown in Figs. 1 and 2 may be dispenseel with. As in Fig. 2, onlythe forward part of the antenna system is shown, the parabolic reflectorbeing omitted for convenience of illustration. In this form of theinvention the reflector dipole 3is carried.- ona yoke,` 50,.' whichforms` an extension of a tubular sleeve 5I which may be slidablyadjusted longitudinally along the outside of the choke sleeve 29. Theposition of sleeve 5| and its yoke extension 50 may be fixed by means ofa set screw 5.2. The yoke 50 andthe sleeve 5| may conveniently be madeof conducting material, such las brass, for since the arms of the yoke5!! are in a plane perpendicular to the dipoles, the structure does notinterfere with the operation of the choke sleeve 29. This structure,however, may also be formed of insulating material. Except for theomission of the polystyrene protective buib and its associated parts andfor the substitution of the structure 50, 5I for the support of theparasitic` dipole 3, the construction of the antenna system shown inFig. 3 preferably follows that of. the antenna system shown in Figs. land 2.

The rounded. end of the end piece of the inner conductor 35 of Fig. 2appears at 53 on Fig. 3. Likewise atti is shown part of the surface ofthe insulating bead shown in cross section at 36 on Fig. 2.

Although it has been determined by experiment that the use of closespacing of the order of one-eighth-wave length between the antennadipole and the reflector dipole, instead of onequarter-wave lengthspacing previously thought to be most desirable, results in betterdirective patterns for the entire system including a parabolicreflector, it is rather diicult to explain on theoretical grounds whythe closer spacing should be superior or even to give an accuratedescription of just how the parabolic reflector is illuminated by thecombination of close-spaced dipoles. It is to be noted at the outsetthat the requirements of an antenna system for illuminating a parabolicreflector and producing therewith a highly directive beam of radiationare to a large extent determined by the maximum allowable diameter forthe parabolic reflector. In a steerable antenna system. and particularlyin one which is regularly moved about for purposes of scanning, itbecomes rather diiiicult to handle very large parabolic reliectors andthis difculty is all the greater when the antenna system is to bemounted on seagoing vessels or on aircraft. Consequently the mostpractical sizes of parabolic reflectors are in the range between` 5I and15 wave lengths,v with reflectors with a diameter of about 8 wavelengths beine rather commonly used.

The shape most desirablel fm` the parabolic reector depends to someextent on the directive properties of the feed. Thus, although beforethis invention it was common practice to use a .parabolic reiiector witha focal length equal to about 0.35 times the diameter, with tbeftype offeed which I prefer, havingr a dipole auxiliary reector about 1A; wavelength in front of the antenna dipole. I nd that a parabolic reflectorwith a focal length equal to about 0.3 timesthe diameter is to bepreferred. If the illumination of the parabolic reector is tapered offtowards the edge of the reflector, there is some lowering of the gain,but such a tapering off of illumination tends to reduce the amplitude ofside lobes, which is an important consideration. In order to obtain adesirable radiation pattern for the entire system including theparabolic refiector, the parabolic reflector should therefore intercepta suicient solid angle of radiation to provide a somewhat taperedillumination. of the parabolic reiiector and the antenna feed sys'- temshould radiate very little laterally or in forward` directions. makingsubstantial angles (such 11 as 15 or more) with the axis of theparabolic reflector.

The radiation pattern of a dipole antenna provided with a parasiticreflector dipole spaced onequarter-wave length away is usuallyrepresented by a curve such as is shown in Fig. 4. The position of thenull corresponds to the direction towards the reflector dipole. Thetheoretical basis of this representation does not take into account anyeffect of the presence of the reflector dipole upon the behavior of theantenna dipole itself and it assumes that the reflector dipole radiatesenergy with a 90 phase difference with respect to the energy radiatedfrom the antenna dipole. It is to be noted that if the reflector dipoleis of an electrical length slightly different from that corresponding toa half-wave length, or if some reaction should take place between thetwo dipoles, the phase of the current excited therein and consequentlythe phase of the radiation from such reflector dipole may be differentfrom that which would be expected from a reflector dipole of anelectrical length of exactly a half-wave length and coupled only byradiation. The foregoing may explain the phenomenon that, as l havefound, the spacing between antenna dipole and reflector dipole producingthe best beam pattern in connection with a parabolic reflector should beof the order of one-eighth-wave length rather than one-quarter-wavelength.

Experiments tend to establish, however, that the radiation pattern ofthe dipole pair arranged in accordance with this invention withapproximately one-eighth wave length spacing between the dipoles is notof the type shown in Fig. 4,

but rather of another type, including a principal lobe of radiation anda small oppositely-directed additional lobe of radiation. Fig. 5 is atheoretical diagram of the calculated radiation pattern for a pair ofdipoles spaced one-eighth wave length apart and excited with a phasedifference of about 155, the intensity of excitation being equal for thetwo dipoles. The solid curve represents the pattern in a planeperpendicular to the dipole (the magnetic plane) and the dotted curverepresents the pattern in the plane which includes the axes of the twodipoles (the electric plane). The dotted pattern is more directive sinceit, as calculated, corresponds to the modulation of the magnetic-planepattern by the characteristic figure 8 radiation pattern of a singledipole in the electric plane.

Fig. 6 illustrates a typical experimentally obtained directive patternmeasurement on a pair of dipoles comprising an antenna dipole and areflector dipole located at one-eighth wave length from the antennadipole, the dipoles being parallel. The solid curve represents thepattern measured in the magnetic plane and the dotted curve representsthe pattern measured in the electric plane. The experimental patternshown in Fig. 6 corresponds to the calculated pattern shown in Fig. 5 inthe existence of sharp nulls in the broad side direction, in the moredirective pattern in the electric plane and in the presence of a smalllobe in a direction opposite to that of the principal lobe. The possiblepresence of additional factors not accounted for in the calculated curveis, however, suggested by the irregularities in the experimental curveand in the increased directivity of both the electric and magnetic planepatterns. It is possible that the thickness of the dipoles may be onesuch factor.

As a result of the small forward lobe of the vfeed system (the principallobe being directed 'lengths or even less.

backwards towards the parabolic reflector), the maximum forward gain ofthe entire antenna system including the parabolic reflector variesrather considerably with the distance between the effective center ofradiation and the vertex of the parabolic reflector, and this variationis periodic and provides a maximum forward gain when the aforesaiddistance is equal to an integral number of half-wave lengths. Thisvariation in gain is particularly noticeable with parabolic reflectorsof small diameter, such as eight wave lengths or less and amounts to afactor of two for a reflector diameter of three wave lengths. Theadjustment of the distance between the effective center of radiation andthe parabolic reflector in accordance with this invention to producemaximum gain brings the radiation from the forward lobe into phase withthe radiation reflected from the parabolic reflector.

A similar direct-forward lobe (sometimes called a back lobe, withrespect to the feed) has been found tc,be exhibited by systems in whichthe auxiliary reflector is a sheet metal disk about a quarter-wavelength in front of the antenna dipole. This may result from the presenceof currents near the edges of the disk. The half-wave point location ofthe effective center of radiation is therefore desirable for suchantenna systems, just as it is in the preferred antenna system having areflector dipole about one-eighth wave length in front of the antennadipole.

The forward radiation, when properly phased as above described by thelocation of the effective center of radiation at a half-wave point,appears to have an additional improving effect upon the directiveproperties of the entire antenna system resulting from the fact that thedirect-forward lobe destructively interferes with the first side lobe ofthe radiation reflected from the parabolic reflector. The beam formed bythe parabolic reflector is narrower than the direct-forward lobe and hasa number of side lobes, some of the closer which fall within the angleoccupied by the direct-forward lobe. It is known from principles ofoptics that the first side lobe on each side of 'the main beam will beopposite in phase to the main beam and that these two side lobes willusually be the most intense of all the side lobes. The direct-forwardlobe, thus, at the same time that it adds to the main beam, counteractsthe most troublesome side lobes, thus improving the overall directivequalities of the beam.

The amount of defocusing resulting from placing the effective center ator near a half-wave point should preferably be kept at a minimum. Ifpossible it would be desirable to provide a parabolic reflector with afocal length equal to an integral number of half -wave lengths.

Antenna systems constructed according to this yinvention areparticularly suitable for steerable directive systems employed for radioecho detection and location service. They may be used either fortransmission or reception and indeed 'also for systems in whichtransmission and reception is accomplished with the same antenna system.Antenna systems according to this invention provide highly directivebeams with narN row beam widths and extremely low side lobe radiation inspite of the fact that the parabolic 'reflector may have a diameter ofonly 4 wave This is a very great advantage for echo detection andlocating systems for it is evident that the parabolic reflector,

13 which according yto Athe -'preferred fforrn of lconstruction itselfsupports the feed line fand antenna system, 'may "be 'mounted --in 'such'Way that it may `berotated about twofdifierent axes, preferably 'atright angles lto 'each other. The mechanical 'arrangements'of ysuchfm'ountingg are well known as they 'have been =use`d `Afor guns,Searchlights and other 'devices fora long time. It is readily apparentlthat rotation aboutboth mounting axes can fbe accomplisheds'in'lultaneously'to producea scanning of adesiredarea or, moreprecisely, of adesired-solid "angle, just as scanning by rotation abouttwo axes has been 'performed in the past inthe `c'asef'of opticalsystenis (e. g., for ltelevision),"lawn sprinklers, 'and so on. Theantenna system -of -the 'present 3invention lends itself readily VtoIconstruction iin .a form `suiiicientl-y sturdy tovenable relatively`.rapid .rot/ation or other movement of the entire antenna system. Ifthe antenna systemlandffeed-is'rotated together with v:the parabolicreflector, of course, some Erotating 'joints will be :necessary lin theyfeed :line which .connects the antenna with the transmittingand/orreceivingsyste'm. Such joints arelnow knownandfhave beensuccessfully operated. The 'arrangement'of the 'feed lineiand thevprovision 'of .'-such joints, although lit may fbe vlan essentialpart'of a continuouslyiscanningltype of y.radio-echo v'detection and.location 'apparatus is not of direct concern "to the practice or theunderstanding of this invention, fand :for that reason is not described:in :detail '.here, :since .it .does not affect the .formation "of a.sharply `directive `beam 'ofi-radiation. :It is,however, .to beunderstood that 2antenna .systems according :to this invention haveadvantagesiparticularly suited for operation in '-a continuouslyscanning .ty-pe of 'detection and location apparatus. This is especiallytrue with respect .to .the provision of low side lobe radiation, forsidesloberesponsein .a continuously scanning system 'which employs avcathode ray type of .indicatordntroduces not only distracting signals'but also an 'undesired Aambiguity because Vthe :indication resultingfrom the side lobe re'sponse Acannot be distinguished .from anindication produced by :the `mainibeam of radiation when pointed in.quite another `di .rection.

What Il'clesire.to1claim and 'secure by Letters rPatent is:

Vl. `A directive lantenna :system including adiipole :antenna connectedto a two-conductorfee'd .line 'adapted :for connection `torrauiioapparatus,

a reflector Iparallel to said 'antenna .land `located rfow/arrfllythereof 'at i a distance not-substantially greater than 0.27 ywavelength, .and a paraboli'c .reflector associated with 'said v.antenna and`located rearwardly 'thereof such a im'anner 'that Itho effective centerof radiation defined .by 'cooperation of saiddipole-fantennaand saidrelflector lies approximatelyfon the'a'nisiof .said parabolic'reflectorat'a distance from 'the vertex "of said parabolic 'reflectorequal :approximately Ito 2. A directive antenna system including a`dipole antenna vconnected *to -a twoec'o'nductor feed line adapted for"connection to-radio apparatus,

va vreflector parallel `to 'said lantenna and located forwardly thereofat a distance y'not :substantially greater than 0.2'7 4Wave length, anda parabolic reflector having a focal length approximately "equalto 'anintegral numberofhalflwave lengths 114 and 'located rearwardly fof said'-.antenna in such a manner that the effective fcenterfo'f radiationdene'd by thecooperation o'f saiddipoleantenna and said "first namedreflector rlies at the focus of vsaid:parabolic reflector.

3.'A directive antennasystem .including a idipole antenna connected tova two-conductor vfeed line adapted for connection'to a radio'fapparatua a'reect'or dipole parallel to'said antenna andf-locatedforwardly thereof atzatdistancenot greater than one-quarter wave length,and a paraboliczrevflectorhaving a focal lengthI approximatelyequal toan integral number of half-wave lengths and located rearwardly of saidvantenna in such a manner 'that `the vmidpoint between the respectivecenters of said dipole antenna and said 1reflector dipole 'lies-at lthefocus of `said Lparabolic reflector.

4. A directive antenna system includingraidipole antenna connected to atwo-conductor feed line adapted for connection to 'radio apparatus, a1reiiector dipole parallel tosaid antenna and located forwardly thereofat azdistance substantiallyzless than one-quarter wave length, and aparabolic reflector associated 'with said antenna .and `1ocatedrearwardly thereof in such-a 'manner that the midpoint between the'respective centers fof said dipole antenna and said lreflector dipoleis located approximately'on theaxis of said ,para-- bolic reector and-ata distance from the vertex of said parabolic reflector of :approximatelyan integral number of half-"wave lengths, .the said integral number lofhalf-'wave lengths being so chosen as to bring saidniidpoint as nearfaspossible'to'the focus `of said-parabolic'reector under the conditionsvherein defined.

5. A directive@ antenna system includingy a dipole antenna `connected toa two-conductor feed line adapted for connection to radio apparatus, a1reector dipole parallelto said antenna an'dflocated forwardly thereofata distance of approximately 'one-eighth wave length, and a parabolicreflector associated with said antenna having a focal length ofapproximately an integral 'number 'of half-wave lengths and'located`rearwardly of .said

antenna in such a mann'er'that the Amidpointbetween the respectivecenters-ofsaiddipole antenna and said reflector dipole lies atthe focusof said parabolic reflector.

6. A directive'antennal system including a dipole antenna connected to acoaxial-'conductor 4feed line adapted for connection to radio apparatusasleeve choke'mounted on theouterconductor of :said'feedline withitsopenend located near .said dipole Yantenna'but not close venough theretoto 'permit the occurrence of corona discharge, a Arenector dipoleparallelto said antenna and located -forwardly thereof at a.distancesubstantiallyless than one-quarter wave length and a iparabolicreflector associated with said antenna and lo- 'cated rearwardly thereofvin such a manner that said feedline passesthrough said parabolic re-7iiector at the vertex of the said .parabolic vre1iec 'tor and issupported at said vertex and .further .that the midpoint'between therespective centers of said dipole antenna and said krefle'ctordipolelies approximately on the axis of said vparabolic yreflector at adistance'lfromsaidvertex equal'to an integral number of half wave.lengths and at a distance fromthe focus of said parabolic reflectorwhich is not greater fthan one-half wave length.

7. A directive antenna system including adip'ole antenna havingarmsof'bulbous yshape connected I"to a co axial-conductor feed vlineadapted Ifor connection to radio apparatus, a reflector dipole slightlylonger than said dipole antenna, parallel to the axis of said antennaand located forwardly thereof at a distance substantially less thanonequarter wave length, a sleeve choke mounted upon the outer conductorof said coaxial feed line having its open end near said antenna but notso near thereto as to permit the occurrence of corona discharge in theoperation of said system and a parabolic reflector associated with saidantenna and located rearwardly thereof in such a manner that themidpoint between the respective centers of said dipole antenna and saidreflector dipole is located approximately on the axis of said parabolicreflector at a distance from its vertex of approximately an integralnumber of half-wave lengths and at a distance from the focus of saidparabolic reflector not greater than one-half wave length.

8. A directive antenna system including a dipole antenna having arms ofbulbous shape connected to a coaxial-conductor feed line adapted forconnection to radio apparatus, a reflector dipole longer than saiddipole antenna, parallel to the axis of said antenna and locatedforwardly thereof at a distance substantially less than one-quarter wavelength, a sleeve choke mounted on the outer conductor of said feed linewith its mouth directed towards said antenna, a prospective vesselenclosing said dipole antenna, said reflector dipole and said sleevechoke and made of material essentially transparent to radiation inconnection with which said antenna system is adapted to operate, thespace enclosed by said vessel co-mmunicating with the space between theconductors of said coaxial conductor feed line, said vessel beingadapted to maintain pressure in the neighborhood of said antenna, and aparabolic reflector associated with said antenna and located rearwardlythereof in such a manner that the midpoint between the respectivecenters of said dipole antenna and said reflector dipole is locatedapproximately on the axis of said parabolic reflector at a distance fromits vertex of approximately an integral number of half-wave lengths andat a distance from the focus of said parabolic reflector not greaterthan one-half wave length.

9. A directive antenna system including a dipole antenna connected to acoaxial-conductor feed line adapted for connection to radio apparatus, areflector dipole parallel to said antenna and located forwardly thereofat a distance substantially less than one-quarter wave length, a vesselof material essentially transparent to radiation in connection withwhich said antenna system is adapted to operate, which vessel enclosessaid dipole antenna and said reflector dipole and is sealed to the outerconductor of said coaxial-conductor feed line, said vessel being adaptedto maintain an internal pressure and having an extended neck adjacent tosaid outer conductor of said feed line, means mounted on said outerconductor of said feed line covering said extended neck in such a manneras to constitute a sleeve choke resonant at the frequency of operationof said antenna system and at the same time to secure mechanically theAsaid vessel upon said outer conductor, and a parabolic reflectorassociated with said antenna and located rearwardly thereof in such amanner that the midpoint between the respective centers of said dipoleantenna and said reflector dipole is located approximately on the axis-of said parabolic reflector at a distance from its vertex approximatelyequal to an integral number of half-wave lengths and at a distance fromits focus not greater than one-half wave length.

10. In a directive antenna system which includes an antenna and acoaxial-conductor feed line connected to said antenna, a vesselenclosing said antenna and sealed to the outer conductor of said feedline made of material substantially transparent at the frequency ofoperation of said antenna adapted to maintain internal atmosphericconditions and having a neck extension adjoining said feed line andmeans mounted on the outer conductor of said feed line embracing saidneck extension to define, in cooperation with said neck extension and aportion of said outer conductor, a choke resonant to the frequency ofoperation of said antenna and adapted to inhibit the formation ofstanding waves on the outside of said feed line, said means being alsoadapted to cooperate to secure said vessel in fixed relation to saidfeed line.

11. A directive antenna system including a dipole antenna connected to atwo-conductor feed line adapted for connection to radio apparatus, areflector dipole parallel to said antenna and located forwardly thereofat a distance substantially less than one-quarter-wave length, aprotective vessel enclosing said antenna and said refiector dipolesealed to and mounted on said feed line, a support attached to saidvessel for supporting said reflector dipole and for maintaining itsposition relative to said antenna, and a parabolic reflector associatedwith said antenna and located rearwardly thereof in such a manner thatthe midpoint between the respective centers of said dipole antenna andsaid reflector dipole lies approximately on the axis of said parabolicreflector at a distance from its vertex of approximately an integralnumber of half-wave lengths and at a distance from its focus not greaterthan one-half-wave length.

12. A directive antenna system including a dipole antenna connected to acoxial conductor feed line adapted for connection to radio apparatus, atransformer in said feed line near said antenna for improving energytransfer between said line and said antenna which comprises a portion ofsaid line of approximately a quarter-wave length having a characteristicimpedance different from that of the rest of said line,a sleeve chokemounted on the outer conductor of said feed line with its lopen endlocated near said dipole antenna but not close enough thereto to permitthe occurrence of corona discharge, a reflector dipole parallel to saidantenna and located forwardly thereof at a distance substantially lessthan one quarter-wave length and a parabolic reflector associated withsaid antenna and located rearwardly thereof in such a manner that saidfeed line passes through said parabolic reflector at the vertex o fthesaid parabolic reflector and is supported at said vertex and furtherthat the midpoint between the respective centers of said dipole antennaand said reflector dipole lies approximately on the axis of saidparabolic reflector at the vertex of the said vertex equal to anintegral number of half-wave lengths and at a distance from the focus ofsaid parabolic reflector which is not greater than onehalf-wave length.

13. A directive antenna systemv including a dipole antenna having armsof bulbous shape connected to a coaxial-conductor feed line adapted forconection to radio apparatus, a transformer in said feed line near saidantenna for improving energy transfer between said line and said antennawhich comprisesa portion of said line of approximately a quarter-Wavelength having a characteristic impedance different from that of the restof said line, a reflector dipole longer than said dipole antenna,parallel to the axis of said antenna located forwardly thereof at adistance substantially less than one-quarter-wave length, a sleeve chokemounted on the outer conductor of said feed line with its mouth directedtowards said antenna, a protective vessel enclosing said dipole antenna,said reflector dipole and said feed choke and made of materialessentially transparent to radiation in connection with which saidantenna system is adapted to operate, the space enclosed by said vesselcommunicating with the space between the conductors of said coaxialconductor feed line, a support associated with said vessel forpositioning and supporting said reflector dipole, and a parabolicreflector associated with said antenna and located rearwardly thereof insuch a manner that the midpoint between therespective centers of saiddipole antenna and said reflector dipole is located approximately on theaxis of said parabolic reflector at a distance from its vertex ofapproximately an integral number of half-Wave lengths l'and at adistance from the focus of said parabolic reflector not greater thanone-half-wave length.

14. A direct antenna system including a dipole antenna having arms ofbulbous shape connected to a coaxial-conductor feed line adapted forconnection to radio apparatus, a transformer in said feed line near saidantenna for improving energy transfer betwen said line and said antennawhich comprises a portion of said line of approximately a quarter-wavelength having a characteristic impedance different from the rest of saidline, a reflector dipole longer than said dipole antenna, parallel tothe axis of said antenna and located forwardly thereof at a distancesubstantially less than one-quarter-wave length, a sleeve choke mountedon the outer conductor of said feed line with its mouth directed towardssaid antenna, a protective vessel enclosing said dipole antenna, saidreflector dipole and said feed choke and made of material essentiallytransparent to radiation in connection with which said antenna system isadapted to operate, the space enclosed by said vessel communicating withthe space between the conductor of said coaxial conductor feed line, asupport associated with said vessel for supporting and positioning saidreflector dipole, and a parabolic reflector associated with said antennaand located rearwardly thereof in such a manner that the midpointbetween the respective centers of the said dipole antenna and saidreflector dipole is located approximately on the axis of said parabolicreflector at a distance from its vertex of approximately an integralnumber of half-wave lengths and at a distance from the focus of saidparabolic reflector not greater than one-half wave length, saidparabolic reflector having a focal length approximately equal to 0.3'times the diameter of said reflector.

15. A directive antenna system including a dipole antenna conected to atwo-conductor feed line adapted for connection to radio apparatus, areflector parallel to said antenna and located forwardly thereof at adistance not greater than one quarter wave length, and a parabolicreflector associated with said antenna and located rearwardly thereof insuch a manner that the effective center of radiation defined by thecooperation of said dipole antenna and said reflector is located at adistance from the vertex of said parabolic reflector equal approximatelyto an integral number of half Wave lengths, said integral number of/half wavelengths being chosen to bring said effective center nearest tothe focus of said parabolic reflector.

LESTER C. VAN ATTA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,054,896 Dallenbach Sept. 22,1936 2,239,724 Lindenblad Apr. 29, 1941 2,407,057 Carter Sept. 3, 19462,413,187 McCurdy et al Dec. 24, 1946 Certificate of Correction PatentNo. 2,486,620 November 1, 1949 LESTER C. VAN ATTA It is hereby certiiedthat error appears in the printed specication of the above numberedpatent requiring correction as follows:

Column 4, lines 54 and 55, for transfoarmer read transformer; column 15,line 29, for the Word prospective read protective; column 16, line 59,for o fthe read of the; line 64, strike out the vertex of the and insertinstead a distance from; column 18, line 19, for conected readoormeoted;

and that the said Letters Patent should be read ascorrected above, sothat the same may conform to the record of the case in the Patent Oice.

Signed and sealed th1s 3rd day of July, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

